A 27-year-old man is bitten while walking through long grass near Tamworth, New South Wales. He brings a photograph of a slender brown snake, but the species cannot be confirmed. Two hours later he has persistent oozing from venepuncture sites, a fibrinogen concentration below the assay limit and an incoagulable blood sample, without ptosis, weakness or dark urine. Which one of the following is the most likely diagnosis?